Manufacture of half tone screens



Oct. 5, 1937.

G. VON KUJAWA MANUFACTURE OF HALF TONE SCREENS 2 Sheets-Sheet 1 Filed Feb. 24, 1933 lnvemor:

By Horney Oct. 5, 1937. G. VON KUJAWA 2,095,015

MANUFACTURE OF HALF TONE SCREENS Filed Feb. 24, 1953 2 Sheets-Sheet 2 12p! & ,ilr'g.9 15 .10 my]! 111 12 1 13 $1,914 fig];

fly/mp1? In ven for m Patented Oct. 1937' UNITED STATES means morscmn or HALF 'roNa soanass Gerhard von Knjawa, Deasau/Anhalt, Germany, asaignor to Agfa Ansco Corporation, Bingham:

ton, N. Y., a corporation of New York Appiication February 24', 1933, Serial No. 658,388 H In Germany February 29, 1932 2 Claims. (01. 95-81) My present invention relates to half-tone screens of the kind used in photo-mechanical reproduction.

One of its objects is to provide a process for the manufacture of improved half-tone screens. Another object are these improved screens. Further objects will beseen from the detailed speciilcation following hereafter. Reference is made to the accompanying drawings in which -Fig. 1 shows the negative of an element of the I half-tone screen,

Fig. 2 shows comparatively the desired courses of the blackening curve of the elements of the half-tone screen for printing processes of the kind of printing half-tone engravings and for flat p fla Fig. 3 shows an arrangement for producing a half -tone screen according to my invention,

Fig. 4 shows a vignette adapted for the production of an element 01 the half-tone screen,

Fig.-5 shows the method of constructing a vignette, and

Figs. 6 to 17 show a set of masks adapted for the production of an element of the half-tone screen.

The conditions for the distribution of blacken-.

} ings and transparencies in the elements of halftone screens which, in the printing and taking process would produce aperfect rendition of the details and the tonal values of the original to be reproduced, are known theoretically. However, a half-tone screen fulillling these conditions could not hitherto be produced.

I have found that it is possible to produce a perfect half-tone screen by multiple projection of the enlarged positive or negative of an element of sented, for instance, in Fig. l. The distance of the perforations of the screen depends on the desired 5- nneness of the screen to be produced. If the plate 8 is arranged at a distance of 80 cm. from the picture I which bears anenlarged element having sides of about 20 cm. in length the perforated screen 2 must be placed at a distance of 0.1 cm.-

60 from the plate 3; the screen has circular perforations of diameter at the most 0.1 mm., the distance between adjacent perforations being 0.25 mm. The enlarged picture of the screen element may be prepared either by drawing, or by photo- 55 graphically reducing a vignette of the shape shown in Fig. 4 beyond the dissolution power of the photographic emulsion, and afterwards strongly enlarging the reduction by soft focus. In the vignette of Fig. 4, the rays have suchdimensions that a distrib'ution of blackening and trans- 5.

parency is "obtained which corresponds to the theoretical conditions required for one screen element. The geometrical construction of such a vignette is seen in Fig. 5.

Instead of making the screen by photograph- 10 ing a screen element with the intercalation of a perforated screen, it may also be prepared, for instance, by photographing a screen element. by the additive method with the aid of a multiplica tor attachment. 15

The reproduction of the true tonal values in pictures taken with the interposition of a screen depends on the gradation of the photographic material and on the variations of gradation which occur in the etching process. The best method 20 of complying fully with these requirements is to operate with a suitably selected. plate 3 or with afipicture I of appropriate dimensions. Thus it is possible to produce a half-tone screen-having any graduation required for reproduction of the orisi- 25 I parencies of a screen element different from that 3 required for reproduction by a flat printing process; if, for instance, the theoretical course of the blackenings on the line AB of Fig. 1 corresponds diagrammaticalLv with curve a of Fig. 2 in the case of iiatprinting, there is used for processes of the 35 kind of half-tone engraving a course of the blackenings corresponding with curve b of Fig. 2 since the dots on the metal picture must be larger at the beginning, because only as the etching process proceeds do they become the correct size. 46

-Instead of using an enlarged picture of a screen element for' producing the screen by multiple projection, there may be used a number of black and white partial pictures of the enlarged screen element, the outlines of these pictures corresponding with lines of equal blackening in the 7 enlarged screen element; these partial pictures are in succession placed at the place of the picture and are photographed on the plate in superimposed relationshi; in the manner above described. The time of exposure must of course be substantially less'than is necessary for complete blackening of the plate and can easily be determined by experiment. A The partial pictures with outlines corresponding with lines of equal blackening 56 phragms placed against a uniformly illuminated white background. In order to ensure continuity (isophotes) may consist, like the hereinbefore described star-like pictures used for the production of the enlarged screen element, of opaque diaof gradation'in the screen to be produced, these partial pictures, in their preferred form, are given outlines which are not sharp, but are fading, hazy or indented. Figs. 6 to 17 represent a set of such masks. The masks must be placed, for the taking operation, successively with their centers at exactly the same point so that the photographic pictures of the diaphragms on the screen will be in concentric superposition.

.By using a set of partial pictures the graduation of the blackening from the center towards the margin can be more easily influenced. When proceeding according to the process described with reference to Figs. 1 to 4, the production of another graduation of shades requires the preparation of a 'difierent picture of the screen element, whether it be in the form'ofa half-tone diaphragm or of a star-like diaphragm. When' using a set of masks which are'photographed successively one upon the other, it is only necessary,'in orderto obtain the same effect, to modify slightly the relative'times of exposure of thedifierent masks. l

Instead of asset of different masks, it is possible to use a mechanically operated diaphragm having a continuously contracting or expanding aperture. Such a diaphragm is knownln theart of reproduction. The diaphragm must, howeverfbe so constructed that'the outlines of itsaperture correspond at each stage approximately with the isophotes of the screen element shown in:Fig. 1.

For the production of a larger number of screens of the kind described, the process oftakin'g multiplied photographic pictures need be performed but once. The screen producedin this manner can advantageously be multiplied by a simple photographic printing process. The photographic reproduction of a screen produced according to the invention involves the advantage that by a proper for this purpose. For the reproduction of a screen made in accordance with this invention, of course, any process known in the printing art may be applied. On a commercial scale the screens may be reproduced, for instance, according to a photomechanical printing process.

WhatIclaimisr I '1. The process of producing a half-tone screen which comprises photographing a vignette under reduction, enlarging the picture obtained, and

projecting the resulting picture through a perforated screen.

2. The process of producing a half-tone screen which comprises projecting of a set of part pictures of an'enlarged picture of a screen element successively one picture upon the other through a perforated screen. I

- GERl-IARD v. KUJAWA.

Furthermore, a photo- 

